Telecommunication and other data networks are rapidly evolving to rely upon optical fiber as the transmission medium. Originally, networks utilizing optical fiber were configured such that fiber simply replaced copper wire on long links so that optical-electrical conversion was required at ends of the optical fiber terminating in the nodes and switches of the network. More recently, all-optical network architectures have been proposed in which optical signals are switched among different fibers of the network without the necessity of converting the optical signal to electrical form.
One popular architecture involves wavelength division multiplexing (WDM) in which multiple optical carriers carrying data signals directed to different destinations are impressed upon a single optical fiber. If such a WDM system is to avoid intermediate conversion to electrical signals, it should include an optical router 10 of the general function illustrated in FIG.1. Two input fibers 12 carry respective sets of wavelength channels .lambda..sub.1 .lambda..sub.2, .lambda..sub.3, .lambda..sub.4 and .lambda.'.sub.1 .lambda.'.sub.2, .lambda.'.sub.3, .lambda.'.sub.4. In the following discussion, differentiation based on wavelength or on frequency will be considered equivalent since they are related through the dielectric constant or refractive index of the local material. The two sets of wavelength channels have nominally the same optical wavelengths but carry different data signals. The router 10 is capable, by mechanisms to be explained later, to selectively route the wavelength channels to different output fibers 14. Because of the nominal equality of the two sets of wavelengths, the router 10 directs, for instance, the wavelength channel .lambda..sub.1 to one of the output channels and the channel .lambda.'.sub.1, to the other.
One type of optical router 10 includes an optical wavelength routing element 20 illustrated schematically in FIG. 2. In this context, an optical routing element is a wavelength division switch, multiplexer or demultiplexer that physically routes optical signals according to their wavelength, the wavelength usually being associated with an optical carrier frequency. A single-moded input waveguide 21 injects light through an optical focussing element 22 into an optical interaction region 28 in which the light spreads and hits a frequency-dispersive element 24. For purposes of this discussion, an optical interaction region is a laterally (horizontally) unconfined planar waveguiding region in which an introduced waveguide field can propagate freely within the plane. A localized mode field introduced into such a region will generally broaden as it propagates through it in the usual mode-expansion manner of unconfined beams. The region may also serve as a space in which fields may be localized, and in which imaging optics are employed to produce a focussed image from a spatially dispersed field.
The light of different wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4 diffracts from or otherwise interacts with the frequency-dispersive element 24 into separate directions so as to couple to respective single-moded output waveguides 26 through focusing elements 25. This figure is meant to be illustrative only. The frequency-dispersive element 24 acts to spatially separate portions of a light beam according to their frequencies. It can assume several other forms, an important one of which is an arrayed waveguide to be described later. The frequency-dispersive element is used for other functions in a WDM network, such as a multiplexer at a multi-wavelength transmitting station and a demultiplexer at a multi-wavelength receiving station.
The frequency-dispersive element in cooperation with the other elements acts as a wavelength filter. Illustrative transmission spectra T(.lambda.) for two neighboring wavelength channels at their respective output waveguides 26 are given in FIG. 3. Each peak 30 has a typically gaussian-like (bell-like) shape with a peak wavelength designed for the design wavelength .lambda..sub.i. The channel spacings .DELTA..lambda..sub.SP are determined by the overall network considerations. For a determined value of .DELTA..lambda..sub.SP, the passband width .delta..lambda. is generally arranged to be large as possible but to be as small enough to provide sufficiently small crosstalk between the channels. At the present, spacings of 0.8 and 1.6 nm are typically used for transmission systems operating at 1.5 .mu.m although specialized devices have been demonstrated with even smaller spacings.
The frequency-dispersive element presents difficulties in a WDM network, particularly one in which a signal may originate from many different transmitters and which then typically travels through many routers. Each of the transmitting lasers emitting at a channel wavelength .lambda..sub.i (and there may be very many of them at geographically distant points) must transmit within a given fraction of the allotted bandwidth .delta..lambda.. However, these lasers tend to drift for a number of reasons including variation in ambient temperature, aging, and other reasons. The bandwidth .delta..lambda. cannot be increased without increasing the wavelength spacing .DELTA..lambda..sub.SP and hence decreasing the total number of channels, that is, the total fiber throughput as system considerations, such as amplifier bandwidths, generally limit the total wavelength span covered by all channels. Even moderate drifts of the laser emission from the peak of a filter transmission curve introduces difficulties. A laser signal at the transmission peak 32 exits the router with larger amplitude than one only slightly down the side of the bell-shaped curve. This difference is multiplied many times as signals traverse many routers, and the difference depends not only upon which laser originated the light but also upon the particular routers the signal has passed through as the filter characteristics vary slightly from one to the next for a number of reasons including variation of ambient temperature, aging, and differences in fabrication, and other reasons so that corrections are difficult when the network interconnects are changing fairly rapidly.
Aspects of this problem have been already recognized. It is desired that the frequency-dispersive element have flat band transmission spectra, such as those illustrated in FIG. 4. Thereby, the laser frequency could drift within the width of the flattened top 36 without effect upon the system. Solutions have been provided for multi-wavelength detectors. Steenberger et al. in "4-channel wavelength flattened demultiplexers integrated with photodetectors," Proceedings ECIO, Apr. 3-6, 1995, Delft, Netherlands, pp. 271-274 (ISBN 90-407-111-9) have disclosed substituting multi-moded output waveguides 26 for the single-moded waveguides in FIG. 2. The optical detectors are then placed at the end of the multi-moded waveguides distant from the frequency-dispersive element. The spectrum of transmission out through a multi-moded waveguide is much flatter than that through a single-moded waveguide. While single-moded waveguides are characterized by the optical power transmitted by a single, fundamental mode, multi-mode waveguide according to the invention is characterized by the total power transmitted through the waveguide. The power transfer into a multi-moded waveguide from a wavelength dispersing region has a much flatter wavelength characteristic than does the power transfer from a similar region into a single-moded waveguide. However, such a solution is not appropriate for communication networks that require long-wavelength transmission to be done over single-moded fiber. The inefficient and unpredictable coupling from multi-moded output guides on the integrated circuit chip to single-moded fiber waveguides introduces excessive loss into the network and may introduce additional wavelength dependence.
Thus, no solution has been found which is appropriate for communications networks, particularly for routers.
Therefore, it is desired to provide a frequency-dispersive element that has a flattened wavelength response but that is efficiently coupled to single-moded fibers.